+
+ If only one of the trees has no branch length, the branch lengths of
+ the other one are ignored with a warning.
+
+ If one (or both) of the trees has no branch length, it is possible to
+ specify a value of 'position' to graft 'y' below the node of 'x'
+ specified by 'where'. In this case, the exact value of 'position' is
+ not important as long as it is greater than zero. The new node will be
+ multichotomous if 'y' has no root edge. This can be solved by giving
+ an arbitrary root edge to 'y' beforehand (e.g., \code{y$root.edge <-
+ 1}): it will be deleted during the binding operation.